Fine-Grained Targets For Laser Synthesis of Carbon Nanotubes

ABSTRACT

A mechanically robust, binder-free, inexpensive target for laser synthesis of carbon nanotubes and a method for making same, comprising the steps of mixing prismatic edge natural flake graphite with a metal powder catalyst and pressing the graphite and metal powder mixture into a mold having a desired target shape.

ORIGIN OF THE INVENTION

This invention was made in part by employees of the United StatesGovernment and may be manufactured and used by or for the Government ofthe United States of America for governmental purposes without thepayment of any royalties thereon or therefor.

FIELD OF THE INVENTION

The present invention relates generally to synthesis of carbonnanotubes, and, more particularly, relates to synthesis of carbonnanotubes using a laser.

BACKGROUND

Carbon nanotubes are allotropes of carbon with a nanostructure that canhave a length-to-diameter ratio of up to 28,000,000:1. These cylindricalcarbon molecules have novel properties that make them potentially usefulin many applications in nanotechnology, electronics, optics and otherfields of materials science, as well as potential uses in architecturalfields. They exhibit extraordinary strength and unique electricalproperties, and are efficient conductors of heat.

One known method of producing carbon nanotubes is laser ablation. In thelaser ablation process, a pulsed laser vaporizes a target in ahigh-temperature reactor while an inert gas is bled into the chamber.The target is a composite of a carbon source (usually graphite or anamorphous carbon powder) and metal catalyst particles (typically acobalt and nickel mixture). Nanotubes develop on the cooler surfaces ofthe reactor as the vaporized carbon condenses. A water-cooled surfacemay be included in the system to collect the nanotubes.

Known art involves pressing and binding targets with a carbon cement(e.g. Dylon G C, Dylon Industries, Inc.). Dylan carbon cement hasgraphite/carbon blend particles that are approximately 200 micron sizedbound with a low surface area lamp black and phenolic resin in furfurylalcohol as a binder. The large particle size inherent in Dylon producesregions of uncatalyzed target that are large compared to the laser spot.Other known techniques involve pressing conventional graphite or carbonpowders that result in structurally weak products. Previous targetrecipes use metal powders that are sold by chemical supply stores, andare specifically selected for their high purity (typically 99.9%) andnot for their particle size.

BRIEF SUMMARY

In at least one embodiment of the present invention, a method for makinga target for laser synthesis of carbon nanotubes comprises mixingprismatic edge natural flake graphite with a metal powder catalyst andpressing the graphite and metal powder mixture into a mold having adesired target shape.

In at least one embodiment, the graphite may have a nominal meanparticle size of less than about ten microns, and typically about fivemicrons. The metal powder catalyst may have a nominal mean particle sizeof less than about one micron, and typically about 0.5 micron.

In accordance with embodiments of the present invention, the metalpowder catalyst may comprise two transition metals, typically nickel andcobalt and typically in about even amounts. The graphite and metalpowder mixture typically comprises less than about twenty percent metalpowder catalyst by weight.

In at least one embodiment, the prismatic edge natural flake graphiteand metal powder catalyst are typically mixed in a ball mill. Thegraphite and metal powder mixture can be pressed into a mold having adesired target shape (generally cylindrical) at a pressure of betweenabout 10,000 pounds per square inch and 20,000 pounds per square inch.Additionally, depending upon the mold material being used, higherpressures could be used to form the targets, with pressures up to100,000 pounds or more a square inch being possible, resulting in usefulvariations in target densities.

In addition to the methods for making a target for laser synthesis ofcarbon nanotubes, as described above, other aspects of the presentinvention are directed to corresponding methods for synthesizing carbonnanotubes and to targets for laser synthesis of carbon nanotubes.)

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a flowchart of a method of synthesizing carbon nanotube, inaccordance with embodiments of the present invention;

FIG. 2 is a perspective view of a target for laser synthesis of carbonnanotubes, in accordance with embodiments of the present invention;

FIG. 3 is a high resolution scanning electron micrograph of a fracturesurface of a binder free carbon nanotube target made according toembodiments of the present invention, wherein the metal catalystsappeared as bright round particles and dispersed very uniformlythroughout the entire graphite matrix.

DETAILED DESCRIPTION

Embodiments of the present invention comprise a method for making atarget for laser synthesis of carbon nanotubes, a method forsynthesizing carbon nanotubes, and targets for laser synthesis of carbonnanotubes. The targets can produce high yield carbon nanotubes via laserablation, such as free electron laser (FEL) ablation.

In at least one embodiment of the present invention, the matrix of thetarget is a prismatic edge natural flake graphite that produces strongstructural pi (π_(p-p)) bonds when pressed at room temperature. Inexemplary embodiments of the present invention, Micro 850 graphite fromAsbury Carbons, with a mean grain size of about five microns, was used,although smaller grain sizes may be desirable. The graphite provides thecarbon source in the target for nanotube formation. The use of prismaticedge natural flake graphite is advantageous because it locks up underpressure without a binder. Other forms of carbon that have previouslybeen used to make targets have to be bound by wet chemistry and do notprovide the small grain size that is desirable. The catalyst used inexemplary embodiments of the present invention can be a powder forgenickel and powder forge cobalt, in substantially equal amounts. Thesemetal powders are small (mean particle size of about 0.5 microns), roundin morphology, highly dispersible, and significantly less expensive thanchemical supply house metal powders. In exemplary embodiments of thepresent invention, Umicore ENP 400 Nickel Powder and Umicore HMP CobaltPowder were employed. FIG. 3 shows a high resolution scanning electronmicrograph of a fracture surface of a binder free CNT target madeaccording to embodiments of the present invention. Uniformly dispersedsub-micron round catalyst particles are visible in the micrograph, whereinter-particle distance is much smaller than a typical laser spot size.Furthermore, the cost of making such a target can be orders of magnitudeless than targets made for conventional carbon nanotube laser synthesis.Target cost is highly significant since the FEL ablation method ofcarbon nanotube synthesis consumes targets at a rate that is at leasttwo orders of magnitude higher than that of conventional lasersynthesis.

Targets of exemplary embodiments of the present invention make use ofpowder forge metals which are very small (approximately 0.5 micron meandiameter) and designed for dispersability as seen in FIG. 3. The purityis typically less (e.g. 99.5%) than chemical supply house metal powders,but the primary contaminant is carbon (the major constituent of thetarget) and is therefore not a disadvantage.

Referring now to FIG. 1, a flowchart of a method of synthesizing carbonnanotube is illustrated in accordance with embodiments of the presentinvention. In exemplary embodiments of the present invention, thegraphite and metal powders are mixed in the desired ratio (typicallybetween about five and about twenty percent metal by weight, althoughthe percent of metal may be as low as about one weight percent ifuniform and smaller catalyst particles are available) (see block 10).Because of the dispersability of the powders, intimate mingling iseasily achieved through a variety of known methods, including low energyball milling. In one embodiment, a mixing jar is filled three-quartersfull with ⅜ inch diameter steel ball bearings, the graphite and metalpowders are added to the jar, and the jar is spun at around 1 Hz on aroller type ball mill overnight (see block 12).

The spun mixture is then pressed in a mold at room temperature with ahydraulic press (see block 14). In exemplary embodiments of the presentinvention, the mold comprises a stainless steel cylinder with a bore thediameter of the desired target, a removable plug in the bottom, and asolid steel ram to compress the charge. The ram may be advanced with acommercial hydraulic press to produce a pressure of about 15,000 poundsper square inch (PSI) in the target, which is generally sufficient tomake a structurally sound compact. In one exemplary embodiment, thefinal targets (element 20 of FIG. 2) were a cylinder with a one inchdiameter, a length of up to 2.5 inches, and a 0.25 inch through-hole(element 22) down the axis to allow for a mounting spindle. Final targetfinishing (hole and surfacing) may be performed on a lathe.

Filling the mold with the graphite and metal powder mixture can bedifficult due to the large volume of the unpacked powders. To addressthis problem, in at least one embodiment, a low pressure packing hopper(which may be made of, e.g., polyvinylchloride (PVC)) may be situatedabove the mold. This hopper is filled with the unpacked powders, thematerial is compacted down the bore into the high pressure mold, thehopper is then removed before the high pressure ram is inserted, andfinal pressing may then be performed in the hydraulic press.

The above described targets may be ablated by a laser to synthesizecarbon nanotubes (see block 16 of FIG. 1). For example, the laser may bea free electron laser, a CO₂ laser, or a solid state laser (such as anNd:YAG laser). In an exemplary embodiment of the present invention,brief (sub-picosecond) laser micro-pulses from a free electron laserablate the target. The beam must be tightly focused (in exemplaryembodiments of the present invention, to a spot size of about 150microns) to achieve ablation threshold. Since the carbon in the targetand the catalyst must be released in constant proportions duringablation, the targets must be fine-grained, relative to this dimension.The above-described targets achieve this desired fine-grainedconsistency, as seen in FIG. 3, a high resolution scanning electronmicrograph of a fracture surface of a binder free carbon nanotube targetmade according to embodiments of the present invention. The metalcatalysts appeared as bright round particles and dispersed veryuniformly throughout the entire graphite matrix.

Systems and methods for laser synthesis of carbon nanotubes, which mayadvantageously use the above-described targets, are described in pendingU.S. patent application Ser. No. 10/188,525, “Synthesis of CarbonNanotubes Using High Average Power Ultrafast Laser Ablation,” filed Jul.3, 2002, U.S. patent application Ser. No. 11/589,011, “Laser AblativeSynthesis of Carbon Nanotubes,” and U.S. Pat. No. 7,663,007 B1,“Apparatus for the Laser Ablative Synthesis of Carbon Nanotubes” whichissued Feb. 16, 2010, the contents of each which are incorporated hereinin their entirety.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

1. A method of making a target for laser synthesis of carbon nanotubes,the method comprising: mixing prismatic edge natural flake graphite witha metal powder catalyst; and pressing the graphite and metal powdermixture into a mold having a desired target shape.
 2. The method ofclaim 1, wherein the graphite has a nominal mean particle size of lessthan about ten microns.
 3. The method of claim 1, wherein the graphitehas a nominal mean particle size of about five microns.
 4. The method ofclaim 1, wherein the metal powder catalyst has a nominal mean particlesize of less than about one micron.
 5. The method of claim 1, whereinthe metal powder catalyst has a nominal mean particle size of about 0.5micron.
 6. The method of claim 1, wherein the metal powder catalystcomprises two transition metals.
 7. The method of claim 6, wherein thetwo transition metals comprise nickel and cobalt.
 8. The method of claim1, wherein the graphite and metal powder mixture comprises less thanabout twenty percent metal powder catalyst by weight.
 9. The method ofclaim 1, wherein mixing prismatic edge natural flake graphite with ametal powder catalyst comprises mixing prismatic edge natural flakegraphite with a metal powder catalyst in a ball mill.
 10. The method ofclaim 1, wherein pressing the graphite and metal powder mixture into amold having a desired target shape comprises pressing the graphite andmetal powder mixture into a mold at a pressure of between about 10,000pounds per square inch and about 20,000 pounds per square inch to form amechanically robust target.
 11. The method of claim 1, wherein pressingthe graphite and metal powder mixture into a mold having a desiredtarget shape comprises pressing the graphite and metal powder mixtureinto a mold having a generally cylindrical shape.
 12. A method ofsynthesizing carbon nanotubes, the method comprising: providing a targetcomprising a mixture of prismatic edge natural flake graphite and ametal powder catalyst, the mixture having been pressed into a moldhaving a desired target shape; and ablating the target using a laser.13. The method of claim 12, wherein ablating the target using a lasercomprises ablating the target using a free electron laser activated forsub-picosecond pulses and focused to a spot size of less than about 250microns.
 14. The method of claim 13, wherein the laser is focused to aspot size of about 150 microns.
 15. The method of claim 12, wherein thegraphite has a nominal mean particle size of less than about tenmicrons.
 16. The method of claim 12, wherein the graphite has a nominalmean particle size of about five microns.
 17. The method of claim 12,wherein the metal powder catalyst has a nominal mean particle size ofless than about one micron.
 18. The method of claim 12, wherein themetal powder catalyst has a nominal mean particle size of about 0.5micron.
 19. The method of claim 12, wherein the metal powder catalystcomprises two transition metals.
 20. The method of claim 19, wherein thetwo transition metals comprise nickel and cobalt.
 21. The method ofclaim 12, wherein the graphite and metal powder mixture comprises lessthan about twenty percent metal powder catalyst by weight.
 22. Themethod of claim 12, wherein the provide target has a generallycylindrical shape.
 23. The method of claim 12, wherein the step ofablating the target comprises ablating the target in an exposed spotwhere the metal powder catalyst is dispersed substantially uniformly.24. The method of claim 12, wherein the mixture of prismatic edgenatural flake graphite and a metal powder catalyst are at least an orderof magnitude smaller than a laser ablated spot dimension.
 25. A targetfor laser synthesis of carbon nanotubes, the target comprising: amixture of prismatic edge natural flake graphite and a metal powdercatalyst, the mixture having been pressed into a mold having a desiredtarget shape.
 26. The target of claim 25, wherein the graphite has anominal mean particle size of less than about ten microns.
 27. Thetarget of claim 25, wherein the graphite has a nominal mean particlesize of about five microns.
 28. The target of claim 25, wherein themetal powder catalyst has a nominal mean particle size of less thanabout one micron.
 29. The target of claim 25, wherein the metal powdercatalyst has a nominal mean particle size of about 0.5 micron.
 30. Thetarget of claim 25, wherein the metal powder catalyst comprises twotransition metals.
 31. The target of claim 30, wherein the twotransition metals comprise nickel and cobalt.
 32. The target of claim25, wherein the graphite and metal powder mixture comprises less thanabout twenty percent metal powder catalyst by weight.
 33. The target ofclaim 25, wherein the target has a generally cylindrical shape.